for Telecom Optical Networks - ist-brighter.eu · Raman Amplification for Telecom Optical Networks...

Raman Amplification

for Telecom Optical Networks

Dominique Bayart AlcatelLucent Bell Labs France, Research Center of Villarceaux

Training day www.Brighter.eu project Cork, June 20th 2008

Raman Amplification for Telecom Optical Networks D. Bayart – Bell Labs France

Raman Amplification for Telecom Optical Networks

Outline of the talk

Raman interaction of Light with the glass Spectral and power characteristics Noise performance and limitations (Rayleigh, noise transfer) System implementation Lumped Raman amplifiers


Effect happens for any pump frequency, polarization dependent Instantaneous effect (not the travel time of the pump along the fiber !) Peak Raman shift = 13 THz (glass phonon energy) Top width = 2.5 THz, can be broadened with multiλ pumping

Raman amplification : Principle

Long interaction fibers Confinement Attenuation at the pump wavelength is paramount

O

O

Si Pump

Signal

( = f pump 13 THz )

Vibrational excitation

Signal

Measured ga in curve : 1 pump @ 1487 nm

0

5

10

15

20

1500 1530 1560 1590 1620 nm

On/o

ff G

ain (dB)

DSF fiber

p p

L

eff

r dB OFF ON P e

A g G

p

⋅ −

⋅ ⋅ = ⋅ −

α

α 1 10 ln 10

/

ρ α 1

= eff L A eff is the Raman effective area of the fiber L eff : Effective length of the fiber expressed as :


Distributed Raman Preamplification

Preamplification (backward pumping : weak gain saturation)

Increase of the signal powers at the EDFA input

(improved signal to noise ratio)

25

20

15

10

5

0

0 25 50 75 100 Distance (km)

Sign

al (dB

)

On/off gain

with Raman

without Raman

Pump

WDM Line fiber


Ca lculated ga in curve : 20 pumps

11,85

11,90

11,95

12,00

12,05

1538 1558 1578 1598 nm

On /off Gain (dB

) ü 2 or 3 wavelengths enough for a 1dB equalization over the Cband

3 to 4 over the C+L band

Ø Semiconductor pumps need polarization multiplexing or depolarization using PANDA fiber at 45 deg.

0

5

10

15

20

25

1500 1550 1600 nm

On /off G

ain (dB)

Equalization over 60 nm

Pump wavelength combination


Fiber efficiency per unit length and per W of pump

0

0,2

0,4

0,6

0,8

0 3 6 9 12 15 18

Ram

an efficiency C R (W

1 .km

1 )

Frequency Shift (THz)

NZDSF NZDSF+

SMF

TeraLight

PSCF

0.68 0.75

0.40 0.51

0.38 13.3THz peak (W 1 .km 1 )

1486nm pumping

C R = 0.23 G on/off / (P p .L eff_p ) C R Max

PSCF : Pure Silica Core Fiber


Normalized Gain Spectra

12 15 18 21 24 27 30 Frequency Sh i ft (THz)

0

0,2

0,4

0,6

0,8

1

0

0,2

0,4

0,6

0,8

1

0 3 6 9 12 15 18 Frequency Sh i ft (THz)

1486nm pump 1400nm pump

PSCF

PSCF

PSCF

Bosonpeak relativeintensity higher with SiO 2 than with GeO 2 13.3THz peak : Gedependent, extends up to > 20 THz Sharp 14.5THz and 18THz peaks specific to SiO 2

CURVES NORMALISED RELATIVE TO 13.3THz PEAK


Fiber efficiency is pump wavelength dependent

0

0 ,2

0 ,4

0 ,6

0 ,8

1

12 15 1 8 21 24 2 7 3 0 0

0 ,2

0 ,4

0 ,6

0 ,8

1

0 3 6 9 12 1 5 1 8 Ram

an efficiency C R (W

1 .km

1 )

Frequency Shift (THz)

1486nm PUMP 1400nm PUMP NZDSF NZDSF+

SMF

TeraLight PSCF

+ 37 % + 21 %

+ 20 %

+ 46 % + 26 %

F Pump/signal/core overlap


Effect of interchannel Raman depletion

Input fiber 10 dBm /channel (linear regime)

5.6 dBm /ch.

100 km #1

#32

Tx Rx

0.7 dB

2.3 dB


Smallfrequencyshift energy transfers

1529 1561 1602

Frequency shift (THz)

Wavelength (nm)

27

22

17 Cband Lband

NZDSF NZDSF+

SMF TeraLight

PSCF

0

0 ,1

0 ,2

0 ,3

0 ,4

0 3 6 9 Ram

an efficiency (W

1 .km

1 )

1569

1486nm PUMP


P u m p

WDM Line Fiber

Intrinsic noise parameter = 3dB (quantum limit)

Equivalent noise figure = the noise figure of an EDFA located in B point and providing with the Raman ON/OFF gain

Equivalent noise figures can be equal or lower than 0 dB

B

Noise figure for Distributed Raman Preamplification


DoubleRayleigh Scattering

• Signal (neglecting the DRS term):

• Simple Rayleighscattered signal:

• Double Rayleighscattered signal: dy L y G y P r L P

dz z y G z P r y P

z G P z P

net L

RS DRS

L

y net S RS

net S S

). ( ). ( . ) (

). ( ). ( . ) (

) 0 ( ). 0 ( ) (

0 → =

→ =

→ =

∫

∫

⇒ DRS noisetosignal ratio at the end of the fibre: R DRS =

P DRS (L)/P S (L)

0 L

Ps (z) r.Ps(z )

P DRS (L)


DoubleRayleigh Scattering (DRS)

• DRS is a delayed copy of the signal => beating noise at detection with the signal by quadratic detection:

• Exists in all transmissions but is more penalizing with distributed Raman amplification: DRS is amplified during its doublepath

• Crucial issue: DRS impairment is a limit to high amounts of Raman gain

• ASE and DRS essentially differ by their spectral distribution:

ASE is constant with wavelength in the range of the signal bandwidth

DRS is a replica of the signal optical spectrum

) ) ( ( 2 ) ( 2 2 ν

ν π ∆ +

∆ ≈

f R f RIN DRS


Electrical measurement of DRS

155 150 145 140 135 130 125 120 115 110 105 100 95

0 1 2 3 4 5 6 7 8 9 10

Frequency (MHz)

RIN (d

B/Hz) .

24dB gain

16dB gain

10dB gain 8dB gain 6dB gain 4dB gain 2dB gain

source

0dB gain


SignalASE and signalDRS beat noises

§ Reference case: NRZ, broad optical filtering

§ General case: any format

elec S r pola sg

ASE 2 2

ASE sg B P N η 4 = σ

S polar sg

DRS 2 2

DRS sg P P η 2 = σ

S ASE ASE 2

ASE sg P P . k = σ

S DRS DRS 2

DRS sg P P . k = σ

depend on: modulation format (signal pattern) optical and electrical filters at reception


Conclusion on DoubleRaleigh Scattering

DRS is a major limitation of the maximum Raman gain that can be obtained in the line fiber Limits Raman advantage in backward pumping

Max gain closed to 23 dB

For all Raman pumping of the line fiber Need for forward pumping

ü See related issues (RIN, …)

Or use of Raman pumping into the DCF ü Increases nonlinear effects in the DCF


Pumptosignal RIN transfer

Raman effect is very fast (femtosecond)

Locally, the intensity fluctuations of the pump are totally

transferred to the signal by gain (dB)

Effect averaged

by counter propagation (backward pumping)

only by chromatic dispersion for forward pumping


Transfer functions

pumping rd for forwa 1 1 2

and

pumping backward for 4

with

1 log 10 ) 10 ln( / 10

log 20 ) ( 2

/

−

=

=

+ −

+ =

P S

P c

S P c

c

OFF ON P S

V V

f

V f

f f G

RIN f RIN

π

α π

α

Assumptions: Distributed Raman amplification: long fiber (50 100km) Moderate Raman gain, moderate pump RIN No pump depletion

Reference: C. R. S. Fludger and al., Electron. Lett., 2001, 37, (1), pp. 1517


135

130

125

120

115

110

1E+ 0 1E+ 3 1E+ 6 1E+ 9 1E+ 12

Frequency (Hz)

RIN (dB/H

z)

Pump RIN

50 kHz 7 GHz

+ 20log(Gonoff/ 4.34)

Transfer functions

With: RIN P =120dB/Hz Gonoff =10dB α P =0.25 dB λ P =1450nm λ S =1550nm

Signal RIN with backward pumping (f c =900 Hz)

Signal RIN with forward pumping SMF (f c =6 MHz) Teralight (f c =18 MHz) may happen with TW and LEAF


Raman amplification : Implementation

Use of wavelength multiplexed Raman pumps

Efficiency depends on fiber type and characteristics

4 pumps (distributed Raman ampli.)

#1 #63

MZ Q

Q

DCF

C

L

DCF

C

L

C

L DCF

x 3

Rx

100 km NZDSF 1x

32

#2 #64

MZ 1x32

40Gb/s 2 15 1

#65 #127

MZ Q

Q

DCF 1x32

#66 #128

MZ 1x32

40Gb/s 2 23 1

PBS

PBS


Stimulated Raman Scattering

17

22

27

5

0

5 In span 5

0 5

27

22

17 Out span

18

13

8

Wavelength (nm) 1525 1605 1427 1565 1485 1439 1450

8

13

18

23 THz

Pretilt ≈3dB

Intrabandtilt compensated but CtoL energytransfer

Raman gain 12dB Cband

10dB Lband

Out span

Pumps OFF

Pumps ON

P

(dBm

)


All Raman pumping schemes

Raman pump

MUX TAP

DCF

DRA in the link:

DRA in the link & DCF:

MUX TAP

Raman pump

MUX TAP

DCF

Raman pump

31dB onoff Raman gain in the link fiber

21.5dB onoff Raman gain in the link fiber

3dB net gain in the DCF (11dB onoff gain)

MUX

Raman pump

MUX

Raman pump


Final impairment of the amplifier on the system:

Achievable distance is proportional to → Parameter C = C noise .C phase (the smaller the better)

Copumping issues to be accounted aside

Criteria for performance assessment (ULH)

in DRS elec ASE noise P R B N C .9 5 *

2 1

+ =

∫ = L

Net phase dz z G γ C 0

) (

Generation of noise

Nonlinear phase

2 / 1 ) ( − phase noise C C


AllRaman transmission of 6 Tbit/s over 6120 km

All Raman amplification (First+Second order)

x3

GFF GFF

+D +D

D

GFF

GFF

Dyn. Gain Equalizer

Link fiber

Tx 149x43Gbit/s Rx

C

L L

C

12 = 6120km

Dyn. Gain Equalizer

Polarization scrambler

Lumped Raman amp.

Lumped Raman amp.


Experimental results with allRaman amplification Spectrum after 6,120km with 149 DPSK channels

In Cband: more odd than even channels Gain excursion close to 10 dB after 6120km OSNR_0.1nm > 16.9dB in L band OSNR_0.1nm > 14.6 dB in C band

Pow

er

10dB

/div.

Wavelength (nm)

1530 1550 1590 1610 1570

Cband Lband


Lumped Raman Amplifier

5 dB

/div

1580 nm 1600 nm 1590 nm

14

16

18

20

22

24

26

10 5 0 5 10 15 20 25 Internal Output Power (dBm)

Internal Gain (dB)

3

4

5

6

7

10 5 0 5 10 15 20 25 In ternal Output Power (dBm)

Internal Noise Figure (dB)

• Use of a specific Raman fiber • e.g. Photonic Crystal Fiber

• Ondemand gain bandwidth and location • Weaker sensitivity to signal gain saturation compared to EDFAs • Robust noise figure in high power input signal regime

Fir st stage

Pump input

Output pump

Second stage Fir st stage

Pump input

Output pump

Second stage


Bibliography

« Erbiumdoped fiber amplifier, Principles and Applications »

Emmanuel Desurvire, Wiley (1994), NewYork

« Erbiumdoped fiber amplifier, Device and System Developments »,

Emmanuel Desurvire, Dominique Bayart, Bertrand Desthieux, Sébastien Bigo,

Wiley (2002), NewYork

« Undersea Fiber Communications Systems »

José Chesnoy Ed., Elsevier, San Diego (2002)

Chap. 4, Optical Amplification, Dominique Bayart

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